Field of the Invention
The present invention relates to an image pickup apparatus including a holding unit for holding signal charge generated in a photoelectric conversion unit of a pixel, a method for controlling an image pickup apparatus, and an image pickup system.
Description of the Related Art
For recent CMOS image sensors, a so-called global electronic shutter with which accumulation periods (exposure periods) of signal charge coincide with one another among a plurality of pixels has been proposed. For example, image pickup apparatuses described in Japanese Patent Application Laid-Open Nos. 2004-111590 and 2015-177349 avoid distortion in an image of a quickly moving object by using a global electronic shutter.
In the image pickup apparatus described in Japanese Patent Application Laid-Open No. 2004-111590, signal charge corresponding to one frame and generated by photoelectric conversion is transferred from photoelectric conversion units to holding units at the same time in a plurality of pixels. With this configuration, accumulation periods of signal charge coincide with one another among a plurality of pixels, and thus, a global electronic shutter can be achieved. In Japanese Patent Application Laid-Open No. 2004-111590, however, signal charge corresponding to one frame and generated by photoelectric conversion is first accumulated in a photoelectric conversion unit. Thus, the photoelectric conversion unit needs to have a large storage capacity (quantity of saturated charge). When the storage capacity of the photoelectric conversion unit is increased, the area of the photoelectric conversion unit increases, and a pixel size increases accordingly.
In view of the foregoing problem, in the image pickup apparatus described in Japanese Patent Application Laid-Open No. 2015-177349, while signal charge generated by photoelectric conversion is accumulated in a photoelectric conversion unit, signal charge corresponding to one frame is divided into parts to be sequentially transferred from the photoelectric conversion unit to the holding unit. This eliminates the necessity for accumulating signal charge corresponding to one frame in the photoelectric conversion unit, and thus, the storage capacity of the photoelectric conversion unit can be reduced.
Even in the image pickup apparatus described in Japanese Patent Application Laid-Open No. 2015-177349, however, while a pixel signal based on signal charge held by the holding unit is being read out, signal charge newly generated in the photoelectric conversion unit cannot be transferred to the holding unit. In particular, since accumulation periods need to coincide with one another among a plurality of pixels in a global electronic shutter operation, while a signal is being read out from the holding unit in at least one of the pixels, accumulation periods cannot start and signal charge cannot be transferred to the holding units in the other pixels.
Consequently, if the readout speed is not high, the accumulation period of signal charge is short so that problems such as degradation of image quality due to a loss of video information based on signal charge or a phenomenon such as jerkiness arise. Although signal charge generated in the readout period can be accumulated in the photoelectric conversion unit, this accumulation requires an increased storage capacity of the photoelectric conversion unit, and thus, a pixel size also increases.